Tertiary Datums and Bolt Circles 5

[Daekar]

Hello all,

I'm a CMM programmer, and at my current place of employment and my last job, I have struggled to find someone who knows the answer to this question. On a circular part with a bolthole circle that has a true position tolerance applied to it, how many datums are required? All the engineers I've spoken to seem to think that one (governing X/Y) is enough, two (one for X/Y and one for Z) is generous, and three (one for X/Y, one for Z, and one for rotation) is overkill to be avoided at all costs. My problem is this: Because of the nature of CMMs, I am forced (as far as I know) to arbitrarily choose one of these holes to set as angularly "perfect" - in essence, make it a tertiary datum - and dimension the rest of the holes off of that hole. Even worse, in parts with more than one bolt-circle, I am faced with the prospect of taking the position of holes from one bolt circle relative to a single hole in another bolt-circle, or doomed to specify several holes as band-aid rotational datums - and try to dimension the entire part this way. It just feels wrong, I'm almost certain that's not the way it's supposed to be, but every other programmer I've met does the exact same thing and never thinks twice about it 'til I bring it up - and the head of our drafting department continually beat around the bush trying not to give me an answer without me realizing he was doing it.

See the attached example drawing for clarification, please ignore missing dimensions, the fact that the diameters of the boltcircles aren't specified, etc... what EXACTLY does this mean? Can those bolt-circle rotate relative to each other? The head of drafting muttered something about "being on centerlines so they can't," but what reference do I use to determine if they have or not if the tabs on the sides aren't datums or aren't present?

 

[fsincox]

pmarc,
Cool, thank.
Frank

 

[fsincox]

pmarc,
Sorry, thanks.

Obviously from the new sketches the ID I referred to earlier was not an ID at all.
Frank

 

[Daekar]

I'm having trouble convincing my supervisor that the title-block tolerance on angles doesn't apply to the clocking relationship between holes defined with basics/TP tolerance and other features specified with standard dimensions. I have a few examples but I don't know how to post more than one picture at a time, so I'll start with the kind of thing we usually get.

When we get a drawing like the one attached to this post, it is typically the intention of the engineer that the tabs and holes clock together. I'm looking for a way to explain why/how this isn't so. I also need to be able to offer an alternative method, so I created the drawing in the next post.

 

[Daekar]

In the drawing attached to this post, I was attempting to use the fact that features given the same datum features in the same order in their true position FCFs are required to be gaged together. However, I'm not sure that I did it correctly. Can you offer any insight? How would you guys convey this information, and how is it specified in the standard? I can't find a thing...

Oh yes, please ignore the extra bolthole circle that doesn't actually go through anything. I had a tertiary datum hole there but decided not to put one in this example, just forgot to remove the circle. Don't want to overload any brains when I have to explain this.

 

[Daekar]

Hrmm... the linkes don't seem to work in the other posts. I bet the filenames are too long. Try these:

http://files.engineering.com/getfil...-464a-a007-d98962455d02&file=Centerlines1.jpg

http://files.engineering.com/getfil...-4812-818d-e2284ba5e289&file=Centerlines2.jpg

 

[pmarc]

Daekar,

Drawing #1:
The way it is specified does not assure that the centerline of a hole is alligned with the centerplane of corresponding tab. These are only someone's intentions that the features clock together. If there is no geometric tolerance between the features specified on the drawing, the features are not related to each other at all.

Drawing #2:
This is exactly the alternative method of specifying clocking between the features. The way you did is correct. I would only delete 9x40deg basic dimension and put EQLSP after 9x .200 dim. for tab width (just as you did for holes) or remove EQLSP from the holes and leave 9x40deg dimension. Take a look at para. 4-19 in Y14.5-2009 std. and figs. 4-40 and 4-41 for confirmation of simultaneous and separate requirements concept.

Hope this helps.

 

[fsincox]

I vote for number 2 also, run thew centerline down to the hole and remove the equally spaced and let the inline 9X cover both. Do you care about the OD and ID, I would have to assume the answer is: "no"?
Frank

 

[Belanger]

I suspect your supervisor is thinking of a rule about "implied zero" dimensions, meaning that things drawn on a common centerline (like the tab and the hole at the 12 o'clock position) are assumed to be centered on each other. That may be true from a drafting perspective, but our real question is about the tolerance on this implied relationship.
I'm not so quick to dismiss what he's saying -- after all, a 90 degree implied angle takes on the title block's angular tolerance, so why not a zero degree implied angle?

But like the other posters, I say that your second picture is much better because now the tabs are called out with GD&T, using the same datum references as the holes. The standard is clear that those are gaged simultaneously (see post #4 above for the the exact rule).
So now your tabs and holes are rotationally "locked" together, with only their respective position tolerances as the allowable clocking error.


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Daekar]

Thanks for the confirmation, guys, I really appreciate it.

Belanger, as far as the "implied zero" rule applying in this case, the problem I have with that is that the exact nature of the relationship is unclear. Does this mean that only the hole and tab directly dimensioned have to be within the title-block angle tolerance? Or that each tab has to be within the title-block anglular tolerance of its corresponding hole regardless of how far out of position the hole is? Do they also have to conform to the given angular dimension between the tabs as well, then? What if there are not the same number of holes and tabs, but four of them happen to be on the same centerlines at the four cardinal angles, how do we determine things then? Do you apply the title-block angle tolerance only to those four? There are too many variables for it to be a reliable method of communicating functional design intentions, as I see it.

Now, if there were the same number of holes and tabs and they were dimensioned with standard linear and angular dimensions with a "typical" notation, then I would say that yes, that they would clock together and the "implied zero" would indeed apply. They would be sacrificing a decent amount of their tolerance zones because they would no longer be round nor potentially subject to MMC bonuses, but I believe this would convey the design intent.

Thoughts on those scenarios?

 

[Belanger]

The 8 X 40º carries the same logic around to all the features, but you're right: We would still have to decide if the tab's angular tolerance should be looking to the tab next to it, or to the hole beneath it?

Ugh -- tell 'em that your proposed solution is the way to go!


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems